Torsion is caused by a twisting force called torque. To calculate torque, equation 1 would be used. Torque that is positive goes in the counter-clockwise direction; negative torque goes in the clockwise direction. If you're confused use your right hand and point your thumb towards yourself, the way your fingers curl represents a positive torque.
All stress caused by torsion is shear stress. For a circular rod, maximum stress is found on the surface, and it decreases linearly to zero as you approach the central axis, refer to the figure below. For objects that are not circular the resulting stress field is different. This will be discussed in another section.
To calculate the stress due to torsion equation 2 would be used.
To calculate the polar moment of inertia for a circular rod equation 3 would be used.
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